Abstract:

Several embodiments of photolithography devices and associated methods of
focal calibration are disclosed herein. In one embodiment, a method for
determining a focus shift in a photolithography system include placing a
microelectronic substrate on a substrate support of the photolithography
system and producing first and second refraction patterns on the
photoresist layer corresponding to first and second grating patterns,
respectively, of a single reticle by illuminating the first and second
grating patterns with an asymmetric monopole source perpendicular to the
first and second grating patterns. The method further includes measuring
an image shift between the first and second refraction patterns on the
photoresist layer and determining a defocus shift of the illumination
source based on the image shift.

Claims:

1. A method for calibrating a photolithography system, the method
comprising:simultaneously illuminating first and second grating patterns
of a single reticle with an asymmetric monopole illumination source
perpendicular to the first and second gratings;producing first and second
refraction patterns on the photoresist layer corresponding to first and
second grating patterns, respectively;measuring an image shift between
the first and second refraction patterns on the photoresist layer;
anddetermining a defocus shift of the illumination source based on the
image shift.

2. The method of claim 1 wherein the photolithography system further
includes an objective lens between the microelectronic substrate and the
reticle, the objective lens having a numerical aperture
(NAobjective), and wherein exposing a photoresist layer
includespassing light with a single frequency (λ) concurrently
through the first and second gratings;refracting the light with the first
and second grating patterns on the reticle, wherein the first grating
pattern has a first pitch (P1) configured as follows: P 1 = λ 2
NA source ##EQU00014## and wherein the second grating pattern
has a second pitch (P2) different than the first pitch (P1);and wherein
determining a defocus shift includes calculating the defocus shift (def)
according to: Δ x = def NA objective * ( σ 0
' + σ 1 ' ) * ( n 2 - ( NA objective *
σ 1 ' ) 2 - n 2 - ( NA objective * σ 0 '
) 2 ) ##EQU00015## where Δx is the image shift, n is the
index of refraction of a medium in which the photolithography system is
disposed, and σ'0, σ'1 are zeroth-order and
first-order refractions of the second grating pattern.

3. The method of claim 1 wherein the first grating pattern is configured
such that a zeroth-order refraction and a first-order refraction of the
first grating pattern are generally equal to each other, and wherein
producing first and second refraction patterns includes producing a first
refraction pattern that does not change with respect to the defocus
shift.

4. The method of claim 1, further comprising producing a light with a
single frequency (λ), and configuring a pitch of the first grating
pattern based at least in part on the frequency (λ) of the light
such that a zeroth-order refraction and a first-order refraction of the
first grating pattern are generally equal to each other.

5. The method of claim 1 wherein the first grating pattern is configured
such that a zeroth-order refraction and a first-order refraction of the
first grating pattern are generally equal to each other, and wherein the
second grating pattern is configured such that a first-order refraction
of the second grating pattern is at least twice as much as a zeroth-order
refraction of the second grating pattern, and further wherein producing
first and second refraction patterns includes producing a first
refraction pattern that does not change with respect to the defocus shift
and producing a second refraction pattern that changes with respect to
the defocus shift.

6. A method for calibrating a photolithography system, the method
comprising:irradiating a reticle with a radiation; andproducing a first
refraction pattern and a second refraction pattern with the reticle,
wherein the first refraction pattern has a zeroth-order refraction angle
and a first-order refraction angle that are generally equal to each
other, and wherein the second refraction pattern has a zeroth-order
refraction angle and a first-order refraction angle that are different
from each other.

7. The method of claim 6 wherein irradiating a reticle with a light
includes irradiating a reticle with a light that is generally coherent
and at an incident angle with respect to the reticle, and wherein the
first refraction pattern has a zeroth-order refraction angle and a
first-order refraction angle that are generally equal to the incident
angle of the illuminating light.

8. The method of claim 6 wherein irradiating a reticle with a light
includes irradiating a reticle with a light that is generally coherent
and at an incident angle with respect to the reticle, and wherein the
first refraction pattern has a zeroth-order refraction angle and a
first-order refraction angle that are generally equal to the incident
angle of the illuminating light, and wherein the first and second
refraction patterns have generally the same zeroth-order refraction
angle.

9. The method of claim 6 wherein the first and second refraction patterns
have generally the same zeroth-order refraction angle, and wherein the
second refraction pattern has a zeroth-order refraction angle
(β'0) and a first-order refraction angle (β'1)
related to each other as follows:sin β'.sub.0.gtoreq.2 sin
β'1

10. The method of claim 6 wherein the first and second refraction patterns
have generally the same zeroth-order refraction angle, and wherein the
second refraction pattern has a zeroth-order refraction angle
(β'0) and a first-order refraction angle (β'1)
related to each other as follows:sin β'.sub.0.apprxeq.9 sin
β'1

11. The method of claim 6, further comprising:exposing a photoresist on
the microelectronic substrate to the first and second refraction
patterns;measuring an image shift between the first and second refraction
patterns on the photoresist layer; andcalculating a defocus shift of the
illumination source based on the image shift.

12. A method for calibrating a photolithography system, the method
comprising:producing a light from an illumination source;producing a
static refraction from the light, the static refraction being
non-shifting with respect to a defocus shift of the illumination
source;producing a dynamic refraction from the light, the dynamic
refraction being able to shift in response to the defocus shift of the
illumination source; andexposing a photoresist on a microelectronic
substrate to the static and dynamic refraction in a single exposure.

13. The method of claim 12, further comprising:creating a first pattern
and a second pattern on the photoresist from the single
exposure;detecting an image shift between the first and second patterns;
anddetermining a focus shift of the illumination source based on the
image shift.

14. The method of claim 12, further comprising:creating a first pattern
and a second pattern on the photoresist from the single
exposure;detecting an image shift between the first and second patterns;
andindicating that the illumination source is out of focus if the image
shift has a finite value.

15. The method of claim 12, further comprising:creating a first pattern
and a second pattern on the photoresist from the single
exposure;detecting an image shift between the first and second patterns;
andindicating that the illumination source is in focus if the image shift
is within a threshold.

16. A photolithography system, comprising:an illumination source
configured to produce a generally coherent light with a single frequency
(λ);a substrate support facing the illumination source, the
substrate support being configured to support a microelectronic
substrate;a reticle between the illumination source and the substrate
support, the reticle having a first grating pattern and a second grating
pattern adjacent to the first grating pattern;an objective lens between
the substrate support and the reticle, andwherein the first grating
pattern has a first pitch (P1) generally equal to a value calculated as
follows: P 1 = λ 2 NA source ##EQU00016## where
(NAsource) is a numerical aperture of the illumination source;
andwherein the second grating pattern has a second pitch (P2) different
than the first pitch (P1).

17. The photolithography system of claim 16 wherein the first and second
grating patterns are in close proximity to each other.

18. The photolithography system of claim 16 wherein the first and second
grating patterns are spaced apart from each other.

19. The photolithography system of claim 16 wherein the illumination
source is off-axis with respect to a normal plane of the reticle.

Description:

[0002]Photolithography is a process commonly used in semiconductor
fabrication for selectively removing portions of a film from or
depositing portions of a film onto a semiconductor wafer. A typical
photolithography process can include spin coating a light-sensitive
material (commonly referred to as a "photoresist") onto the surface of
the semiconductor wafer. The semiconductor wafer is then exposed to a
pattern of light that chemically modifies a portion of the photoresist
incident to the light. The process further includes removing one of the
incident portion or the non-incident portion from the surface of the
semiconductor wafer with a chemical solution (e.g., a "developer") to
form a pattern of openings in the photoresist on the wafer.

[0003]The size of individual components in semiconductor devices is
constantly decreasing in the semiconductor industry. To accommodate the
ever smaller components, semiconductor manufacturers and photolithography
tool providers have produced higher numerical aperture (NA)
photolithography systems using smaller wavelengths (e.g., 193 nm). The
high NA has improved the resolution of the photolithography systems, but
this enhancement in resolution comes at the expense of the overall focus
budget. As a result, the focus and/or exposure control must be very
precise to avoid reducing product yields. Therefore, the focus of
photolithography systems must be calibrated accurately and efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004]FIG. 1 is a schematic view of a photolithography system configured
in accordance with an embodiment of the disclosure.

[0005]FIGS. 2A and 2B are schematic top views of a calibration reticle
useful in the system of FIG. 1 in accordance with embodiments of the
disclosure.

[0006]FIG. 3 is a partially schematic cross-sectional view and top view of
the photolithography system of FIG. 1 in accordance with an embodiment of
the disclosure.

[0007]FIG. 4 is a partially schematic top view of the photolithography
system of FIG. 1 in different focal conditions in accordance with an
embodiment of the disclosure.

[0008]FIGS. 5A-C are schematic top views illustrating simulation results
for calibrating the system of FIG. 1 in accordance with an embodiment of
the disclosure.

DETAILED DESCRIPTION

[0009]Various embodiments of photolithography systems for processing
microelectronic substrates and associated methods of calibration are
described below. The term "microelectronic substrate" is used throughout
to include substrates upon which and/or in which microelectronic devices,
micromechanical devices, data storage elements, read/write components,
and other features are fabricated. Such a microelectronic substrate can
include one or more conductive and/or nonconductive materials (e.g.,
metallic, semiconductive, and/or dielectric materials) that are situated
upon or within one another. These conductive and/or nonconductive
materials can also include a wide variety of electrical elements,
mechanical elements, and/or systems of such elements in the conductive
and/or nonconductive materials (e.g., an integrated circuit, a memory, a
processor, a microelectromechanical system, etc.) The term "reticle"
generally refers to a plate with areas of varying transparencies that
allow light to shine through in a defined pattern. The term "photoresist"
generally refers to a material that can be chemically modified when
exposed to electromagnetic radiation. The term encompasses both positive
photoresist configured to be soluble when activated by the
electromagnetic radiation and negative photoresist configured to be
insoluble when activated by light. A person skilled in the relevant art
will also understand that the disclosure may have additional embodiments,
and that the disclosure may be practiced without several of the details
of the embodiments described below with reference to FIGS. 1-5.

[0010]FIG. 1 is a schematic view of an embodiment of a photolithography
system 100 configured in accordance with an embodiment of the disclosure.
FIGS. 2A and 2B are schematic top views of a reticle 108 useful in the
system 100 of FIG. 1 in accordance with an embodiment of the disclosure.
As shown in FIG. 1, the system 100 can include an illumination source
102, a reticle 108, an objective lens 107, and a substrate support 104
arranged in series about an axis 101. The substrate support 104 can be
configured to carry a microelectronic substrate 106 having a layer of
photoresist 110. In one embodiment, the substrate support 104 can be
stationary. In other embodiments, the substrate support 104 can move
laterally (as indicated by the arrow A) and/or vertically (as indicated
by the arrow B) relative to the reticle 108.

[0011]The illumination source 102 can include an ultraviolet light source
(e.g., a fluorescent lamp), a laser source (e.g., an argon fluoride
excimer laser), and/or other suitable electromagnetic emission sources.
The illumination source 102 can also include condensing lenses,
collimators, mirrors, and/or other suitable conditioning components (not
shown). In several embodiments, the illumination component 102 can
include an asymmetric monopole source with a numerical aperture
(NAsource) defined as follows:

NAsource=sin α (Equation 1)

where α is a maximum incident angle between emitted waves from the
illumination source 102 and the axis 101.

[0012]In certain embodiments, the illumination source 102 can be
configured to produce a generally coherent illumination at a single
frequency (λ). The phrase "coherent illumination" generally refers
to illumination with waves that arrive at a receiving component (e.g.,
the reticle 108) at approximately the same phase angle. In the
illustrated embodiment, the illumination source 102 is offset from the
axis 101 by an angle α. In other embodiments, the illumination
source 102 can also be generally centered about the axis 101 and at least
partially incoherent. The phrase "partially incoherent" generally refers
to waves of illumination that do not arrive at a receiving component
completely in phase. For example, the illumination source 102 can have a
finite physical size and generate waves incident upon the reticle 108
with different phase angles. In further embodiments, the illumination
source 102 can also be configured to generate illumination at multiple
frequencies.

[0013]The reticle 108 can include a substrate having a first grating
pattern 108a adjacent to a second grating pattern 108b. The term
"grating" generally refers to a regularly spaced collection of generally
parallel slits, channels, openings, and/or other transparent or
semi-transparent elements. For example, in one embodiment, the reticle
108 includes a substrate (e.g., quartz) carrying a layer of a generally
opaque material (e.g., chromium) with certain portions removed to form
parallel slits, channels, openings, and/or other patterns on the
substrate. In other embodiments, the reticle 108 can include a first
layer of a semi-opaque material (e.g., molybdenum) and a second layer of
a generally opaque material (e.g., chromium). Certain portions of the
first and/or second layers may be removed form parallel slits, channels,
opening, and/or other desired patterns on the substrate.

[0014]The first grating pattern 108a can have different characteristics
than the second grating pattern 108b. For example, the first grating
pattern 108a can have a first pitch different than a second pitch of the
second grating pattern 108b. The term "pitch" generally refers to a
distance between two adjacent grating elements. In other examples, the
first and second grating patterns can have different transparencies
and/or other characteristics. Several embodiments of the reticle 108 are
described in more detail below with reference to FIG. 2A and 2B.

[0015]The objective lens 107 can be configured to project the illumination
refracted from the reticle 108 onto the photoresist 110 of the
microelectronic substrate 106. The objective lens 107 can have an
objective numerical aperture defined as follows:

NAobjective=sin β (Equation 2)

where β is an angle for a field of view of the objective lens 107.
Equation 2 assumes that the objective lens 107 and the illumination
source 102 are disposed in the same medium. One skilled in the art would
understand that if these components are disposed in different media,
their numerical apertures can be adjusted accordingly with the indices of
the media.

[0016]The system 100 can be configured such that the zeroth-order
refraction from the reticle 108 can be offset from the axis 101. For
example, in certain embodiments, the illumination source 102 of the
system 100 can be at least partially incoherent. In other embodiments,
the illumination source 102 can be coherent but off-axis with respect to
the reticle 108. In all of these embodiments, the system 100 can have a
partial incoherency represented as follows:

σ = NA source NA objective ( Equation 3 )
##EQU00001##

In other embodiments, the system 100 can be configured such that the
zeroth-order refraction of waves from the illumination source 102 can be
generally aligned with the axis 101, and the first and second orders can
be offset from the axis 101.

[0017]FIGS. 2A and 2B are schematic top views of a reticle 108 useful in
the system of FIG. 1 in accordance with embodiments of the disclosure.
The reticle 108 can include a generally circular plate 109 that carries
the first and second grating patterns 108a and 108b. As shown in FIG. 2A,
in certain embodiments, the first and second grating patterns 108a and
108b can be in close proximity to each other (e.g., generally abutting
each other). In other embodiments, as shown in FIG. 2B, the first and
second grating patterns 108a and 108b can be separated by a distance (D).
In further embodiments, the first and second grating patterns 108a and
108b can also be side-by-side and/or have other geometrical arrangements.
The grating patterns 108a and 108b can be generated using machining,
etching, and/or other suitable techniques.

[0018]FIG. 3 is a partially schematic cross-sectional view of the reticle
108 and top view of the photoresist 110 of FIG. 1 in accordance with an
embodiment of the disclosure. For clarity, FIG. 3 only shows the reticle
108 and partial planes 112a and 112b on the photoresist 110 corresponding
to the first and second grating patterns 108a and 108b, respectively. The
first and second grating patterns 108a and 108b are shown separated from
each other by a distance for illustration purposes. The orientation of
both the first and second gratings 108a and 108b is generally
perpendicular to the illumination source 102. The first grating pattern
108a can have a first pitch P1, and the second grating pattern 108b can
have a second pitch P2. In other embodiments, the reticle 108 can include
other grating patterns.

[0019]The first pitch P1 of the first grating pattern 108a can be
configured to produce a zeroth-order refraction and a first-order
refraction (denoted σ0 and σ1, respectively) that
have generally the same value, as shown in FIG. 3. For example, the first
pitch P1 can have a value calculated as follows:

P 1 = λ 2 NA objective σ = λ 2
NA source ( Equation 4 ) ##EQU00002##

when the zeroth-order refraction is spaced apart from a center of
refraction by generally the same distance but opposite direction as the
first-order refraction:

σ0=σ1 (Equation 5)

where

σ 0 ≡ sin β 0 sin β
and σ 1 ≡ sin β 1 sin
β . ##EQU00003##

Thus, the zeroth-order refraction angle β0 generally equals the
first-order refraction angle β1. As a result, the grating
equation of the zeroth-order refraction for the first grating pattern
108a can be reduced to:

sin α=sin β0 (Equation 6)

The grating equation for the first-order refraction for the first grating
pattern 108a can be written as:

As a result, the zeroth-order refraction angle β0, the
first-order refraction angle β1, and the incident angle α
generally have the same absolute value. In certain embodiments, the
zeroth-order refraction σ0 and the first-order refraction
σ1 are configured to coincide with the partial incoherency
σ of the system 100. Thus, substituting Equations 1-3 into Equation
8 can yield Equation 4. In other embodiments, the partial incoherency
σ of the system 100 can be larger than the zeroth-order refraction
σ0 and the first-order refraction σ1.

[0020]The second pitch P2 of the second grating pattern 108b can be
configured to produce a zeroth-order refraction and a first-order
refraction (denoted σ'0 and σ'1, respectively) that
have different absolute values, as shown in FIG. 3. In certain
embodiments, the zeroth-order refraction σ'0 can be at least
twice as large as the first-order refraction σ'1. In other
embodiments, the zeroth-order refraction σ'0 can be at least
five times as large as the first-order refraction σ'1. In
further embodiments, the zeroth-order refraction σ'0 can be at
least nine times as large as the first-order refraction σ'1.

[0021]According to the grating equation for the second grating pattern
108b, the zeroth-order refraction angle β'0 generally equals
the incident angle α. As a result, the grating equation of the
zeroth-order refraction can be reduced to:

sin α=sin β'0 (Equation 9)

The grating equation for the first-order refraction for the second grating
pattern 108b can be written as:

As a result, a designer can select values for the zeroth-order refraction
and the first-order refraction (σ'0 and σ'1,
respectively) and calculate the second pitch P2 according to Equation 11.

[0022]During focus calibration, the microelectronic substrate 106 is
placed onto the substrate support 104 with the photoresist 110 facing the
objective lens 107. Then, the illumination source 102 illuminates the
reticle 108 at an incident angle α. The reticle 108 refracts the
incident waves onto the objective lens 107 with the first and second
grating patterns 108a and 108b. The objective lens 107 redirects the
refracted waves onto the photoresist 110. The microelectronic substrate
106 can then be developed using a suitable chemical solution (e.g., a
mixture of metol, phenidone, dimezone, and hydroquinone). The chemical
solution can remove a portion of the photoresist 110 to yield first and
second refraction patterns corresponding to the first and second grating
patterns 108a and 108b. The developed microelectronic substrate 106 can
then be inspected with a critical dimension scanning electron microscope
(CDSEM) and/or other suitable metrology tool. The metrology tool can be
used to measure an image shift (Δx) between the first and second
refraction patterns on the photoresist 110.

[0023]Without being bound by theory, it is believed that the defocus phase
shift (Δφ) of the zeroth-order and the first-order diffraction
of a grating pattern is governed by the following formula:

where def is the focus shift, and n is the medium index of fraction. The
derivation of Equation 12 is described below with reference to FIG. 4,
which is a partially schematic top view of the photoresist 110 of FIG. 1
in different focal conditions.

[0024]As shown in FIG. 4, if the illumination source 102 is best focused
onto the photoresist 110, spherical waves from the illumination source
102 can produce a first circle 402 with a focus center 401 and a first
radius of r. If the illumination source 102 is defocused, then different
spherical waves from the illumination source 102 can produce a second
circle 404 with a second focus center 401' and a second radius of R. The
amount of focus shift (def) is the distance between the first and second
focus centers 401 and 401'. Thus, the difference between the first and
second circles 402 and 404 at an incident angle of α can be
expressed as follows:

[0025]As discussed above, the first grating pattern 108a is configured
such that σ0=σ1. Thus, based on Equation 12, the
defocus phase shift (Δf) of the zeroth-order refraction is equal to
that of the first-order diffraction:
Δφ1=Δφ0. The image shift (Δx) of the
first grating pattern 108a can be calculated by performing Fourier
transformation of the zeroth and the first order diffraction pattern as
follows:

Δ x = ( Δ φ 1 - Δ
φ 0 ) * p 2 π = 0 ( Equation 17 )
##EQU00010##

As a result, the image of the first grating pattern 108a would not shift
at the defocus plane.

[0026]The image of the second grating pattern 108b, however, would shift
at the defocus plane. It is believed that the defocus phase shift
(Δφ) of the zeroth-order refraction and the first-order
diffraction of the second grating pattern 108a is as follows:

As a result, the focus shift (def) at the imaging plane (i.e., the
photoresist 110) can be derived based on the measured image shift
(Δx'), and the first grating pattern 108a can be used as a
reference for determining the image shift.

[0027]In certain embodiments, the derived focus shift (def) can then be
used to adjust the focus curvature of the illumination source 102,
movement of the substrate support 104, and/or other operations of the
system 100. In other embodiments, if the image shift has a finite value,
it can be indicated that the illumination source 102 is out of focus. In
other embodiments, if the image shift is within a predetermined threshold
(e.g., 5 nm), then it can be indicated that the illumination source 102
is in focus.

[0028]Several embodiments of the system 100 can determine a defocus phase
shift of an illumination source more efficiently and accurately than
conventional techniques. According to one conventional technique, two
reticles with different patterns are sequentially exposed to derive an
defocus phase shift. Such sequential exposures are time-consuming and
susceptible to device drift and/or other environmental influences,
resulting in unreliable measurements. Several embodiments of the system
100 can determine the defocus phase shift with only one exposure. As a
result, the amount of time required for calibrating the system 100 can be
reduced, and the reliability of the calibration can be improved.

[0029]A specific example of applying the calibration process is described
with reference to FIGS. 5A-C, which are schematic top views of the first
and second refraction patterns 508a and 508b under various defocus
conditions. Specific values of illumination wavelength, medium index of
refraction, numerical apertures, and other parameters were used for
illustration purposes only. One skilled in the art will understand that
these parameters can also have other desired values based on the
particularities of a photolithography system.

[0030]In the illustrated example, the following parameter values were
used:

λ=193 nm

n=1.43664

NAobjective=1.34

P1=0.080(μm)

P2=0.144(μm)

σ0=σ1=σ'0=0.9

σ'1=0.1

[0031]As shown in FIG. 5A, when the illumination source 102 is
substantially focused on the photoresist 110, the image of the first and
second grating patterns 508a and 508b are generally aligned with each
other. As a result, the image shift (Δx) equals to zero. As shown
in FIG. 5B, as the illumination source 102 is defocused, the image shift
(Δx) has a finite value of 24 nm. Using Equation 20 and the
parameters listed above, the focus shift (def) can be calculated to be
about 0.05 μm. As shown in FIG. 5C, as the illumination source 102
becomes more defocused, the image shift (Δx) now has a larger value
of 48 nm. Again, using Equation 20 and the parameters listed above, the
focus shift (def) can be calculated to be about 0.1 μm. Even though
the defocus of the illumination source 102 is described above as
proceeding in only one direction, the calibration process described above
can also be applied when the illumination source 102 is defocused in the
opposite direction.

[0032]From the foregoing, it will be appreciated that specific embodiments
of the disclosure have been described herein for purposes of
illustration, but that various modifications may be made without
deviating from the disclosure. For example, many of the elements of one
embodiment may be combined with other embodiments in addition to or in
lieu of the elements of the other embodiments. Accordingly, the
disclosure is not limited except as by the appended claims.